The present invention relates to a digital video system for distributing analog video signals from multiple video sources, such as cameras, VCRs or the like, to multiple video receiver terminals in the form of digital video information.
This application is based on Japanese Patent Application No. 9-137773, filed May 12, 1997, the content of which is incorporated herein by reference.
In TV broadcast stations, a video distribution system is used which distributes simultaneously analog video signals from multiple TV cameras to multiple monitors. Depending on circumstances, these monitors may be distributed over multiple studios. A director chooses video information to be broadcast while watching the monitor screens. In this specification, unless otherwise specified, video is defined as containing audio.
In FIG. 1, there is illustrated a schematic of a conventional video distribution system, which is equipped, as video sources, with multiple cameras 10.sub.1 to 10.sub.m each with a motor-driven universal head, a motor-driven focusing/zooming mechanism, and a microphone. These cameras are unmanned ones that are installed on the roof of a building or the like. An analog video signal from each of the cameras 10.sub.1 to 10.sub.m is supplied over a video channel 12 to a corresponding one of video distributors 14.sub.1 to 14.sub.m, each of which then distributes the analog video signal of the corresponding respective camera to first and second routing switchers 16 and 18. Each of the first and second routing switchers 16 and 18 is a unidirectional analog routing switcher. The first switcher 16 has two separate matrix switches: one for video signals and one for audio signals, whereas the second switcher 18 has three separate matrix switches: one for video from camera to monitor, one for audio from camera to monitor, and one for control signals from monitor to camera. A video signal and a audio signal are each transferred over the corresponding switch. The first routing switcher 16, which is adapted for video broadcasting device, couples one of its several inputs to its single output. A single analog video signal thus selected from multiple analog video signals is supplied to a broadcasting device 20.
The second routing switcher 18, which is adapted to distribute video signals to monitors 22.sub.1 to 22.sub.n installed in a studio, selectively couples its inputs to its outputs. In response to a select signal from each of the monitors, the second routing switcher 18 sends a desired video signal to that monitor. In this case, a certain video signal may be supplied simultaneously to the monitors 22.sub.1 to 22.sub.n. Each of the monitors is equipped with a loudspeaker and a remote controller for controlling the panning, focusing, zooming or the like of a camera that is shooting a picture being displayed from the monitor side. The control signals from the remote controllers of the monitors are supplied over the control signal matrix switch in the second routing switcher 18 and a control channel 24 to the universal heads and the focusing/zooming mechanisms of the cameras 10.sub.1 to 10.sub.m.
With this conventional system, users can operate the second routing switcher 18 to switch among video signals for each of the monitors 22.sub.1 to 22.sub.n and, while watching the monitor screens, can operate the remote controller to control the universal heads and focusing/zooming mechanisms of the cameras from the monitor side so as to adjust camera angle, focusing, and angle of view.
However, the conventional system divides one video signal into two: one for broadcasting and one for monitoring. In addition, a separate matrix switch is required for each of video and audio signals. Thus, large-scale wiring facilities are needed. To install more cameras and monitors, therefore, it is required to add routing switchers or their ingredients, matrix switches, and wirings to accommodate additional signals. For renewal, the system has to be shut down for a long time and a great cost is involved.
Moreover, each of the routing switchers is unidirectional. Thus, the transfer of control signals from monitors to cameras cannot be made over the same routing switch and lines as video or audio signals. The use of separate wirings for control signals increases the scale and cost of the system.
On the other hand, one method of distributing video signals to multiple monitors is coaxial cable-based broadcasting (CATV). With this system, it is relatively easy to increase the number of monitors, but it is difficult to install more cameras as with the conventional system described in connection with FIG. 1. In addition, the system is also basically unidirectional. The transmission of control signals from monitors to cameras thus requires separate wiring facilities, increasing the system scale and cost.
Furthermore, one system that has a feature of distributing data to multiple terminals is a computer-based LAN (Local Area Network). It will be expected to transfer video signals over the IEEE 802.3 LAN that is now widely used. However, this LAN is a band sharing type of communications system that employs carrier sense, multiple access, collision detection (CSMA/CD) techniques. When a collision is detected at the time when a data transmission is made, the transmission is stopped, and a retransmission is made at a later time. If an attempt is made to simultaneously transmit video signals from multiple locations, other signals than one will undergo delays. This will cause disturbances in pictures displayed on monitors. That is, video transmissions require so strict real-time integrity as not to cause transmission delays and frame rate variations during transmission. The time interval between the moment that one frame of picture information arrived and the moment that the next frame of picture information arrives depends on the number of frames per unit time of original picture information. If the number of frames is not retained, reproduced pictures will suffer from loss of picture quality. That is, unlike data transmission LANs which have some tolerance in real-time integrity, video-oriented LANs require very reliable time management; otherwise, picture quality may suffer considerably. If, when the users adjust camera angles and angles of view while watching the monitor screens, responses from the cameras are delayed, they will have difficulty in the adjustment work.
In addition, since the conventional LANs use optical cables, hardware cost is increased significantly and difficulties are involved in installing the cables.